It's a kind of distributed Ponzi scheme that lacks the ordinary intent and centralized accounting associated with them.

It's different from startup in that a startup's shares increase in value, at least theoretically, on the likelihood of future dividends from the company. Startups can have bubbles or be Ponzi schemes too, but if they're indeed successful, they can earn money, and that money (or at least the bet that it will materialize) is a basis for sound investment by passive third parties who have capital.

Bitcoins increase in value only if you can sell them to future owners who are willing to pay more than you did. It's not quite a Ponzi scheme, but it has all of the important characteristics of one: old money is paid back from the new, and the value increases only on the hope that you can involve more new money.